Consumers continually pressure integrated circuit manufacturers to provide devices that are smaller and faster, so that more operations can be performed in a given amount of time, using fewer devices that occupy a reduced amount of space and generate less heat. For many years, the integrated circuit fabrication industry has been able to provide smaller and faster devices, which tend to double in capacity every eighteen months or so.
However, as integrated circuits become smaller, the challenges of fabricating the devices tend to become greater. Fabrication processes and device configurations that didn't present any problems at a larger device size tend to resolve into new problems to be overcome as the device size is reduced. For example, gate lengths of metal oxide semiconductor type transistors have been dramatically shortened over the last several years. The reduction in the gate length has introduced a wide variety of new issues that engineers and scientists are working to overcome.
As a specific example, gate electrode materials such as polysilicon have typically been patterned using a photoresist mask and a dry etch. However, photoresist tends to degrade to a certain degree during a dry etch, such as erosion of the photoresist at the edge of the mask. While such edge anomalies had relatively little effect on a relatively wide gate electrode, an edge anomaly such as this tends to have a commensurately greater effect on a relatively narrow gate electrode.
Thus, other materials besides photoresist have been used as masking materials for such critical etch processes. For example, silicon dioxide has been used as a so-called hard mask for the patterning of gate electrodes and other structures within integrated circuits. While the use of these other materials as hard masks tends to reduce the problems with anomalies at the edges of the mask pattern, other problems have arisen from their use. For example, when the etch for which the hard mask has been formed is completed, it is typically desirable to remove the hard mask. However, such hard mask materials tend to be more difficult to remove than photoresist. Thus, in removing the hard mask material from the integrated circuit, other underlying layers are often damaged or otherwise negatively affected. To continue the specific example introduced above of a silicon dioxide hard mask that is used to pattern a polysilicon gate electrode, removing the silicon dioxide hard mask often etches the underlying gate insulation layer, which is also typically formed of a silicon dioxide.
What is needed, therefore, is a method by which a hard mask can be used to pattern structures on an integrated circuit, without unduly damaging underlying layers when the hard mask is removed.